It is common in the art of information communication to transmit and receive signals which are divided into a plurality of signal lines, each of which is a successive line of pixel signal values taken from a two-dimensional information field. For example, television signals are divided into a plurality of horizontal line intervals, each of which is a successive line of light intensity values from a two-dimensional video field, or image, composed of such values. Facsimile signals are also composed of signal lines, each of which represents one line of varying light intensity values scanned across the image being transmitted. Linear or time delay and integrate (TDI) imaging charge-coupled devices (CCDs), such as those used for airborne reconnaissance, commonly generate a signal which is divided into a plurality of signal lines. TDI imaging CCDs are commonly constructed out of a plurality of closely spaced, parallel TDI CCD shift registers built on a photoelectric semiconductive substrate. Such devices have been designed to have an optical image scanned across the surface of their substrate along the length of their TDI shift registers and to have those shift registers clocked in synchronism with the motion of the optical image. This is done so that electrons which are freed under one of the TDI shift registers by the photoelectric effect of light in a given portion of the optical image are attracted into a charge packet that will be moved along by the TDI shift register in conjunction with that given portion of the optical image. When a charge packet reaches the end of its associated TDI shift register it is fed, in parallel with all of the other charge packets which have reached the end of their associated TDI shift registers at the same time, into an output CCD shift register. The output shift register then rapidly shifts out all of the charge packets fed to it, so as to provide a series of charge packets, the variable charge levels of which correspond to the variable light intensity of a line taken from the two-dimensional optical image scanned across the TDI imaging CCD.
When transmitting signals which are divided into a plurality of signal lines, each of which is a successive line of pixel signal values taken from a two-dimensional information field, it is often desirable to communicate each pixel signal value with considerable accuracy. For example, TDI imaging CCDs can easily be constructed which produce pixel signal values with an accuracy that requires eight binary bits to represent (a greater or lesser number of bits can also be used). Representing each pixel signal value with so many binary bits has the benefit of conveying a greater amount of information about the image from which the signal was derived, but it has the unfortunate effect of increasing the amount of time or channel capacity required to transmit the signal and of increasing the amount of memory required to store the signal if such storage should be desired. Therefore it is desirable to find a way to compress the amount of data required to represent such a multiline signal without greatly decreasing the meaningful information it conveys concerning the image from which it was derived.
One method of data compression which has been used in the past is to transmit, instead of the absolute value of a pixel to be transmitted, the value of the difference between the value of that pixel and the one that preceded it. This method results in a data compression because the values of pixels which are next to each other are usually very close in value, particularly if the pixels are closely spaced relative to the size of the features on the image from which they were derived. Thus the number of bits required to represent the difference between the value of such neighboring pixels is less than that required to represent the absolute value of such pixels.
In such a method, the absolute value of a first pixel is transmitted in an uncompressed form, so that the absolute value of a second, succeeding pixel value can be reconstructed by adding to the value of the first pixel the differential value transmitted as the compressed representation of the second pixel. Once the absolute value of the second pixel value has been reconstructed, the absolute value of a third pixel succeeding the second can be reconstructed by adding to the reconstructed absolute value of the second pixel the differential value representing the third pixel. This process of reconstructing absolute pixel value can be repeated for each of a succession of pixel values.
Unfortunately, there are problems involved in using a data compression method which merely transmits the difference between a pixel value and that of its preceding pixel value. For one thing, such a data compression technique is very subject to noise. If the value of a reconstructed pixel value has a given error, whether it be due to noise in the derivation of the original absolute pixel values, in the transmission and reception of the data compressed signal, or in the reconstruction of the absolute pixel values, all succeeding reconstructed pixel values derived from that erroneous pixel value will have their values offset by the full amount of the given error. As a result, one badly reconstructed pixel will cause all succeeding reconstructed pixels after it to also be erroneous until a new absolute pixel value is transmitted in uncompressed form.